


Indigo Flush

by Barely_Meeting_Expectations



Category: Undertale (Video Game)
Genre: Accidental Plot, Awkward Grillby, Body Dysphoria, Body Worship, Cinnamon Roll Sans, Depressed Sans, Ecto-Genitalia (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Eventual Relationships, Eventual Romance, Eventual Smut, Flame dick lmao, Fluff and Angst, Fluff and Smut, Gaster mention, Gaster references, Grillby's, Just helpin a bro out man, Loss of Virginity, M/M, Mating Cycles/In Heat, Monster Heat, Plot What Plot/Porn Without Plot, Post-Traumatic Stress Disorder - PTSD, Sans is in heat and he dont even know it lmao what a noob, Self Confidence Issues, Self-Hatred, Shy Sans, Soul Sex, Veteran Grillby, W. D. Gaster is an asshole and we need to stop him, accidental feels, and i use that term very loosely/casually, i dont really know what im doing with this, i just i love them ok, its 12 am and im adding every tag possible to this dumb thing because i dont want to sleep, sans/grillby - Freeform, sansby - Freeform
Language: English
Status: In-Progress
Published: 2016-08-28
Updated: 2017-02-24
Packaged: 2018-08-11 12:54:02
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 7,200
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7893226
Author URL: https://archiveofourown.org/users/Barely_Meeting_Expectations/pseuds/Barely_Meeting_Expectations
Summary: <blockquote class="userstuff">
              <p>“Sans, have you considered… that you might be... in heat?”<br/>“hate to break it to ya, flame boy, but skeleton’s can’t do that. im just as disappointed as you are, i know. the bone zone would have made a great pun to use someday. too bad it don’t exist, right?”<br/>Grillby damn near dropped the glass he’d spent the last half an hour polishing.<br/>-<br/>Or, in which Sans learns that skeletons can go into heat, and Grillby awkwardly offers his assistance. (This was supposed to be pure smut but the first chapter is literally just plot buildup lmao im so sorry)</p>
            </blockquote>





	1. it's not fear, hothead. (In which Grillby makes a quaint discovery, much to the dismay of his skeletal friend.)

**Author's Note:**

> Because I’ve been hella depressed and unmotivated lately and need to do something lmao. I also have yet to write like straight up porn and im telling myself I can do it and also I’m always a SLUT for this whole monster heat thing so were in for a sinful ride lmao  
> And also? There just, needs to be more Sansby? Like in general? Plz understand.

Because I’ve been hella depressed and unmotivated lately and need to do something lmao. I also have yet to write like straight up porn and im telling myself I can do it and also I’m always a SLUT for this whole monster heat thing

 

Grillby first noticed Sans’ odd behavior when the skeleton didn’t sit at his usual barstool on his sentry duty break.

Instead, the stout little monster beelined for the booth seat closest to the window, occasionally wiping the magical prespiration off of his forehead with the sleeve of his (damp?) blue hoodie. He appeared to be sweating much more than usual, desperately pressing his face against the cool glass, much to the dismay and interest of onlookers outside the resteraunt. Grillby’s first thought was that he was being more active to satisfy his brother’s insatiable need for physical activity - which didn’t make the least bit of sense, seeing that the two skeleton brothers didn’t necessarily have muscles to build up in the first place. But the fire elemental knew that Sans would go great lengths to keep Papyrus happy, so perhaps that was the reason.

It was then the bartender noticed that Sans hasn’t requested his usual, just the bottle of ketchup (the refrigerated one, even; Sans never cared about the temperature of his ketchup, but he was adamant for something nice and cold).

While delivering said chilled condiment, Grillby inquired about the dampened hoodie, to which the skeleton replied “made a snow angel with paps” or something of that nature. There were no puns in the sentence. Papyrus was also on duty at this time, so there was no way he would stop looking for humans just to make a snow angel, even if it were for his brother. He was a very dedicated monster. So then, what else could cause Sans to act so feverish?

When he finally spotted the deep blue flush in the skeleton monster’s cheeks, spread across his face like butter on toast (come to think of it, Grillby was getting hungry), he eventually understood what was going on.

Still, he decided to play dumb anyways, just in case he unintentionally made things awkward, or managed to mistake a common fever with monster heat. Grillby also wasn’t known to be very talkative in general, so him asking these questions no doubt already rose the short skeleton’s suspicions.

“Have you managed to get yourself sick, Sans?” The elemental asked concernedly, approaching his favorite customer as he vigorously polished a glass to keep his hands busy.

Said skeleton tore his unfocused, glossy gaze away from the glass and glanced at Grillby before staring down at the ketchup in his (twitching?) hands.

“nah.” Sans shrugged sluggishly. “can’t get sick, grilbs. i probably just overused my magic or something. happens every once in a while, it’ll go away in a week or so. nothin im not used to by now. it’s just, i don’t like paps seein me so…”

Sans sloppily gestured to himself with a distraught huff. 

“…like 'this'… he gets so worried about me, about everything, so it’s for the best i just avoid him until i’m feelin better. usually i just lay in the snow until it goes away but paps wasn’t havin that this morning, so.”

Sans resorted back to staring at the window, sighing as he pressed himself against it, leeching the glass’s low temperature into his overheating bones. It appeared, to Grillby at least, that Sans hadn’t the faintest clue what was happening to him. So he decided to drop the “playing innocent” shenanigans and decided to drop the question on him.

Grillby cleared his throat nervously.

“Well, Sans, have you considered… that you might be on your… cycle?”

Sans looked a little out of it when he asked the question and Grillby suddenly regretted even suggesting it, until a look of realization crossed Sans’ face.

“hm? …ooh. heheh.” His smile dropped slightly. “hate to break it to ya, flame boy, but skeleton’s can’t do that either. im just as disappointed as you are, i know. the 'bone zone' would have made a great pun to use someday. too bad it don’t exist, right?” He went back to staring outside, fixated on the falling snow.

Grillby damn near dropped the glass he’d spent the last half an hour polishing. His makeshift jaw was agape.

“Sans, that’s not… who ever told you such a preposterous thing?” The bartender spat out before he could stop himself.

The little skeleton recognized the irritation in his voice without even looking at his baffled expression and closed his eyes.

“...no one important. but they were one of my own species, and an excellent scientist, at that. i like to believe he would have known what he was talking about, when it came to… stuff like that, at least.”

Wow. Um. Grillby didn’t know how to go about this one without sounding like a know it all jerk or a horny creep. Did this mean that Sans didn’t know anything about his own anatomy or magical reproductive abilites? Did he even know what going into heat fully entailed? Had he even heard of soul sharing?

The bartender slowly set down the glass he was polishing (Sans didn’t even notice) and wrung his hands together as he cautiously pressed on.

“... Well, surely you must know… all monsters go through it, regardless of species or origin. Especially ones with large or rare magical reserves. It is common with every monster, for reproductive purposes. It is extremely dangerous for the bearer to contain their magic while in this process. That monster could have very well been lying to you, Sans.”

Eyes still closed, Sans nonchalantly shrugged as if he could care less.

“doesn’t matter. i guess you could say paps an i are a bit of an exception, i suppose.” 

“Oh? And why’s that?”

A dark, saddened expression momentarily shined through his features, before reverting back to his carefree, trademark grin. 

“dunno.”

Still no puns. Grillby pressed on.

“You and Papyrus have some of the strongest magical reserves I’ve ever seen, Sans. I’m surprised it doesn’t hit you both like a train, honestly. I would love to speak with whomever told you this, because skeletons also most certainly become sick as well as go into heat. When I was serving in the war, several of the skeleton monsters I knew suddenly fell ill in the camps and I-”

“he’s dead.”

Grillby’s flame flickered blue. Sans gradually sat up and rested his face in his hands.

“…the guy who told me so… he died a long time ago.”

Grillby suddenly felt like the biggest piece of shit in the world.

“Oh… I am so sorry-”

“haha!” The bellowing, genuine laughter both startled and concerned Grillby out of his mourning stupor. “don’t be. that guy was an asshole anyway.”

That made him feel a little less shitty.

The fire elemental cleared his throat and waited for his flames to stop flickering before gently placing a hand on San’s shoulder. He nearly flinched away.

“My apology still resides, but might I ask, Sans… would you believe the scientist whom you say yourself is an asshole, or the bartender that has seen more than enough in his many years of experience to confirm your fear?”

Sans suddenly grew defensive, using one of his hands to swat away Grillby’s with an irritated expression.

“hey, it’s not fear, hothead.”

“Then why are you supressing natural urges?”

Sans squeezed the ketchup bottle harder. The red condiment oozed out over the side and over his fingers and he didn’t seem to notice, or care. He deeply inhaled through his nose (or lack thereof??) and then out of his mouth. Grillby couldn’t help but to notice that his hands had begun to shake again.

“...alright. alright, fine, say it is heat, grillby. what does that change for me or papyrus, huh? it’s the same pain in the ass only now it’s got a name, and we both still get to suffer through it at random intervals every year - because lemme tell ya, there’s no way i’m letting that gaudy-ass, glittery-ass, smart-ass robot lay a finger on my brother… or anyone else, now that i think about it.”

Grillby’s soul dropped to his feet. Is that what Sans honestly thought of it as?

“Well, that is incredibly unhealthy for the both of you, Sans.”

“you got a better idea, doctor grilbs?” Sans sneered.

'Get laid', Grillby desperately wanted to say, but quickly realized that there were still a few customers left in the bar, and he hoped Sans didn’t say that part about him going into heat too loud. Instead, the bartender sighed and straightened his bowtie.

“Well, your brother is another story for another time.” He spoke gently and quietly, hoping it would have an adverse effect on Sans’s sudden frustration. “In the meantime, have you possibly considered finding yourself a temporary mate? At least until after you’ve-”

Sans interrupted the elemental with a hearty laugh and he was honestly taken aback when the skeleton wiped actual tears from his eyes. Some of the other customers looked a bit startled as well, before leaving a tip and quickly departing the bar. After all, when Sans laughs, it meant there were bad jokes in their immediate vicinity, or somebody was gonna have a bad time.

“grilbs, haha jeez… ol buddy ol pal, you got me, that was a good one!” Sans continued to giggle.

Grillby was not laughing. He shot Sans a stern look, small amounts of smoke beginning to seep out the sides of his head.

“you’re joking, right? that was a joke, right? hot damn, fireball!”

“Do I look like I am joking? Sans your wellbeing-”

Sans held up his hand and cut the taller monster off. He stopped laughing.

“look, grillby. i’m gonna give it to you straight. i’m a four foot tall skeleton with 1 hp and absolutely zero redeeming qualities. i spend all of my freetime getting borderline shitfaced at a bar and constantly falling asleep at random places with no warning or reason. i drink ketchup straight out of the bottle and there is literally a trash tornado in my bedroom, right next to my collection of left socks. i don’t know where the right ones go, but just the left ones are good enough for me i guess. there isn’t a monster within a fifty foot mile radius at least, that would want to bone me. out of pity, even.”

Sans immediately shot a crapload of ketchup directly into his mouth, downing it like there was no tomorrow as Grillby stood in awe at the skeleton’s words. Again, Sans resorted to looking out the window and away from Grillby’s shocked gaze, acting like his words meant nothing.

After what seemed like forever, the fire monster spoke.

“Sans, that’s not true.”

“facts are facts, flame boy. never said you had to like them.”

“Sans, you can’t-”

“why bother lookin when you know the answers gonna be no.”

“I-”

“hell, why bother lookin at all when it’s all just going to be re-”

“Sans!” Grillby slammed his hands down against the table angrilly.

Sans flinched backward, releasing his ketchup bottle and hitting his spine against the back of the seat as he unintentionally threw himself away from his friend’s violent outburst.

Upon noticing his actions, Grillby quickly withdrew his arms and inspected the table for damage (which, thankfully, was minimal). He went to apologize when Sans let out a deep sigh, shooting the taller monster a saddened look.

“... grillby, i know this might not come as a shock to you, but i don’t… like… anything about me… there’s, really not much to like… and if i can’t even bring myself to do that much, then… who else would be able to? who else would even be willing to…”

He tore his gaze away from Grillby’s pained expression and back outside.

“... sometimes, i feel like ga-the scientist who told me that skeleton monsters were incapable… honestly just told me so because he knew that i would never be able to-”

Sans was engulfed in an instant, long fiery arms encasing him and yanking him out of the booth, holding him tightly to Grillby’s chest.

“Sans, you are far more magnificent then you set yourself out to be. You are a clever, intelligent, and very loving monster who deserves nothing but the very best. Anyone who tells you otherwise obviously doesn’t know you well enough, and that’s on them. Not you.”

Sans had no words. So, tears would work fine too, he supposed.

After the two hugged for an extended period of time, which neither of the two seemed to mind, Grillby pulled back and cleared his throat.

“Now, um… from what I understand, this means that you have not… been with anyone in a sexual nature?”

If elementals could sweat, Grillby would have doused out his own flames by now. This wasn’t too awkward to offer, he tried to tell himself. Thank god the last of his customers had left the bar by now, freaked out by Sans’s boisterous, unnatural laughter.

“If, uh, if you are comfortable with it, I…” Grillby put much thought into how he would say "I would bang you in a heartbeat" in a professional and friendly manner.  
“I would be willing to help you rid of this problem sooner… it would be much faster and much healthier than waiting it out, as you normally do.” Now flustered, the fire monster averted his gaze nervously. “Of course, I understand if you wouldn’t want to. Only if you are willing to, I will not pressure you to do anything you don’t want to-”

A skeletal hand shot up and cut him off.

“no, i-i want to! it’s just, you know, i… i mean, i don’t wanna seem spineless or anything, but i just don’t know what it means, tibia honest. i-i’ve never soul shared with anyone before, or… anything like that, as you could imagine, so…”

So he really didn’t know anything about monster reproduction, and was told that it and anything of that caliber was merely impossible. Grillby was suddenly happy that this scientist was dead, otherwise Grillby would have had to kill him himself.

“Generally, Sans, soul sharing is reserved for monsters who have spent many heat cycles together. It occurs naturally as it becomes more comfortable. That is far down the road, and is usually the doorway to the birth of a new monster soul, should you choose to reproduce that is. If you are not used to going into heat or are not physically prepared to bear children, then the heat primarily focuses on… channeling the build up of magic in alternative ways.”

At least Sans partially realized what that hinted at, Grillby noted, as his face flushed indigo.

“grilbs, like i said before, i’m just a skeleton. i literally don’t-”

“There are many monsters born without sexual organs. Their magic will temporarily create a replica of sorts as an outlet upon arousal.”

Sans' eyes shrinked into pinpricks of uneasy light, looking to and fro as the flush spread further across his cheeks.

“... jeez, upon what?”

Grillby smiled mischeviously.

“That’s the fun part.”


	2. cat's outta the bag now. (In which Sans' heat is dealt with and all he can respond with is stupid cat puns.)

**Notes for the Chapter:**

> YALL THIRSTY AS FUCK LMAO I’m sorry this took so long but here its unedited and 14 pages long lmao bye  
> There’s just not enough of this gross content out there so welp here you go I guess it’s time for the both of us to uh regret thing  
> Also I’ve never written smut so if you’re disappointed that makes two of us lmao I hate how this came out I cry they deserve better and I spent sooo long writing this lmao  
> Also this chapter kept reminding me of Empire Ants by Gorillaz??? but uh, give it a listen its kinda fitting

The two monsters awkwardly but unanimously agreed that it wasn’t a good idea to take care of Sans’ “situation” in front of Grillby’s bar windows for all of Snowdin to see (it wasn’t necessarily off of Grillby’s list of potential to-dos, of course, should this turn out to be a regular thing.) The fire elemental also suggested that Sans needed to loosen up a bit first before anything particularly frisk-y happened - the skeleton laughed at that one - and invited him upstairs to his living quarters to make him feel more comfortable.

“nice digs, fireball.” Sans commented as they stepped into the cozy living space together. There wasn’t too much room, seeing that Grillby lived by himself, but it the crackling fireplace and the faint scent of earthy smoke made Sans feel much more at home than the booze and ketchup-scented atmosphere of the bar. He found himself unintentionally staring at the numerous portraits of various colored flame monsters lining the walls, before unintentionally dissociating and detaching himself from reality completely.

“Would you like something to drink, Sans?” The fire monster offered politely.

‘Anything but water,’ Grillby silently thought to himself, idly wondering if skeleton monsters needed water to survive at all. He knew by now that they could drink - ridiculous amounts even, he’d seen Sans down more ketchup bottles than he could count. He would love to see how much Sans could take in terms of alcohol… later…

Sans was incredibly focused on the floorboards for some reason (looked as if they were swept and freshly mopped, maybe polished, too) and shot his head up when he heard his name, having completely missed the offer.

“... what?” he asked quickly. “wait what did you say? sorry.”

Grillby smiled widely at his cluelessness. What a goofball.

“You still appear to be quite anxious. Remember what I said, you do not have to do this if you are uncomfortable. The snow idea would be ‘snow problem’, if you would prefer to do that instead.”

Although it would be quite a problem for him, should he have to be the one to apply the stuff; he just really wanted to make another pun for Sans because he knew he liked them. And the authentic smirk he got in response to them was always rewarding.

“no no, it’s not- eyyyy, ice-y what you did there, pretty slick, grillbs. seriously though, it’s not that, grillby, i mean i…”

Sans wrung his skeletal hands together nervously, his tired eyes darting back and forth from the (really nice, genuinely impressive) floor and Grillby’s glasses in an uncertain manner.

“i mean, i want to do this. i really do, but… i just don’t know if it’s a good… when you first started askin me all those questions earlier, do you remember what i said when i got all… eugh, uhh…”

At a lack for words (and not wanting to use the phrase “catty” out loud, seeing as he knew a cat monster and he didn’t want to be disrespectful), Sans made an angry cat-like noise and clawed his skeletal digits through the air - and if that wasn’t the most adorable thing Grillby has ever seen, then maybe his brief mental image of Sans dressed up as a cat monster was. He reminded himself to convince Sans to wear cat ears later, if anything to satiate his curiosity. And maybe he would even draw whiskers on him in his sleep or something. That would be cute, up until his brother saw it, then in the off chance Papyrus didn’t agree, it would just be hilarious.

Quickly getting off track, Grillby shoved the amusing image out of his head and nodded in response, albeit with a knowing smile.

“Well, that is most certainly one way to describe it, yes. You told me that what you were feeling was not fear.”

Sans chuckled uneasily.

“... heh, yeah i did, huh? Well i lied…that’s exactly what it is…” He looked away, as if he were ashamed.

“it’s fear…”

And Grillby had known it was fear from the start. He chose not to say anything though, as he cautiously set a comforting hand upon the skeletons shoulder, slow enough this time so that Sans wouldn’t flinch.

“There is absolutely nothing wrong with that. Understandable, even, considering the fact that you’ve never had such an experience before, and have been told that it were impossible. This is something you never want to rush or force.”

Small beads of sweat trickled down the side of Sans’ skull and he nervously cleared his nonexistent throat for what seemed like the fifth time that day.

“r-right… it uh, stupid question, but… it won’t like… hurt, or anything… will it?”

“No Sans, it will not hurt. Quite the opposite, in fact. But I think it would be better for you if you were in a more familiar setting. Papyrus won’t be returning home for quite awhile, won’t he? Perhaps we should go to your house instead. Does that sound better? Would you be more comfortable then?”

Sans was temporarily at a loss for words and racked his skull for a response, graciously taking about a full minute to put a sentence together.

“...you don’t- you wouldn’t mind doing that?” He managed to stutter as the blue blush on his cheeks continued to grow again.

 

Grillby shook his head, his flames following the motions swiftly.

 

“Not at all. It’s a slow day, since you scared all of the regulars away. And friends come first, don’t they?”

 

Grillby laughed at the pun he made there. That was pretty slick of him. Too bad Sans didn’t quite catch onto it, but that would change soon enough if he could help it.

 

“oh yeah, huh. sorry about that. yeah, we can chill at my place if you’re cool with it. my room’s kind of a trash can, though.”

 

“I’m sure I’ll live. Shall we go?”

 

“sure. but uuhh, c’mere. i know a shortcut.”

 

Sans closed his eyes and spread his arms wide, gesturing for Grillby to come closer with a happy grin. Without any hesitance, the fire elemental stepped forward and returned the embrace gently; his breath hitched when Sans firmly wound his arms around his waist.

 

“i would hang on a little tighter than that if i were you.” Sans suggested, before a gust of wind surrounded them and Grillby quickly clung onto Sans for dear life.

 

Another rush of wind overtook his senses, and suddenly Grillby’s feet were no longer on the ground. A sudden blackness took over them. Garbled, jumbled voices and the sound of static echoed throughout the vast nothingness before him - he swore he could see a figure extending its gnarled, slimy digits from the depths of the darkness, reaching for him, and then-

 

His feet landed on the bedroom floor with a solid thud. Sans gently released him and chuckled to himself.

 

“welcome to my humble abode.” Sans hopped onto his bed, his eyes still closed with a relaxed demeanor about him.

 

A few silent seconds passed and when Sans reopened his eyes, Grillby was gawking at him in awe.

 

“... geez, grillbs, you’re lookin right through me, what’s up?”

 

“I just- I had no idea you could teleport, Sans. No wonder it’s like you appear out of nowhere. That’s quite a rare and talent, even in boss monsters.”

 

A clouded look took over Sans’ expression.

 

“yeah.” He whispered. “i know.”

 

Grillby couldn’t help but wonder what he was thinking about when his eyes went blank and his smile went still. What was going through his mind when he got unbearably quiet and a darkness engulfed his eyelights? Whatever it was, he decided early on that he didn’t like it. He liked it much better when there was a mischevious twinkle in his eye and a laugh behind that grin. He liked it better when his face was flushed blue and not sheet white because of whatever awful memories were playing through his head.

 

Perhaps he would ask sometime.

 

But now was not that time, and the fire elemental would be damned if he let Sans go today without a smile at the very least.

 

“Well, if you’re ready, I suppose we should begin.”

 

Sans stiffened out of the corner of Grillby’s eye as the fire elemental briskly began removing his shoes - he didn’t want to get dirt all over Sans’ stained mattress - before delicately pushing Sans down against the bed and lying beside him.

 

The bartender had decided early on that going slow (or rather, slower than usual, especially for an unpracticed monster in heat) would be best for both of them - not only because Sans had never slept with anyone, but because he was pretty sure that Sans wasn’t used to physical contact in general. As far as he knew, the only other monster he would let touch him was Papyrus, and that was in terms of occasional hugs and piggy-back rides. Sans didn’t usually open up to anyone about anything, really.

 

Only a few seconds passed with them being still in such close proximity before Sans began to quiver nervously. The taller monster snaked his fiery arms around the other and enclosed him in a warm embrace, seeping heat through Sans’ jacket and shorts.

 

“Is this alright?” Grillby spoke softly.

 

Sans shrugged and cleared his non-existent throat.

 

“... we’re uhh… hugging, grillbs.”

 

“Actually, I believe the term is ‘cuddling’.”

 

“that’s just hugging lying down, my guy.”

 

“You still seemed a little tense. And cold. Sans?”

 

“yeah?”

 

“May I kiss you?”

 

“uuuhhhhhh…”

 

Sans shifted uncomfortably in Grillby’s arms. He readjusted himself so that he could look at the elementals glasses.

 

“i mean… we can sure as hell try grillbs, but neither of us have, like… actual… lips, sooo...”

 

“Lips are overrated. The ones on our faces, at least.”

 

“but how are we supposed to- wait what?”

 

“Nothing. Would you like to give it a go, then?”

 

“yeah man. but like i said a thousand times i don’t know what the heck im doing-”

 

Sans was cut off as the flame monster suddenly straddled him and, with no warning, caressed the side of his skull and pressed their faces together.

 

It was a strange sensation, being kissed by a flame elemental. Seeing that both of them didn’t have typical lips, he assumed this couldn’t have been like a kiss shared between humans. This felt like something much more intimate, their magic intermingling with each other tentatively. Sans could feel heat buzz through his teeth and into his bones, spreading his blush even further. It tickled and slightly singed his makeshift lips (or rather, the humorous lack thereof) and sent a shiver down his spine. After a few moments, Sans closed his eyes and allowed the curious flames to creep through his smile. Buzzing, intoxicating magic flowed through his skull and he let out a quiet, hushed moan.

 

Taking this as a good sign, Grillby deepened the kiss and the passionate flames crept deeper into Sans, intertwining with Sans’ blue magic and producing a strange sizzling sound. The skeleton certainly didn’t need air, but he found himself panting anyway.

 

The indigo flush on his cheeks burned brightly. Grillby could feel the need in the other’s magic intensify, as if waking up from a long sleep, urging him to press forward.

 

Grillby sat up and swiftly pulled his fireproof shirt up and over his head, neatly folding it and hanging it over the headboard with a little too much care, considering his location.

 

Sans stared in awe at the elemental monsters literally glowing form above him. Brilliant shades of bright red, dusty orange and sunlight yellow drifted peacefully throughout his being. His light radiated throughout the room and it was one of the most beautiful things the skeleton had ever seen. Sans imagined that this must be what the sun looked like.

 

“whoa... lookin hot, grillbs.”

 

“Thank you. Generally, the participants of coitus undress themselves beforehand. It is supposed to make things much easier for all parties involved.”

 

(“And intimate,” Grillby wanted to add, but didn’t want to make the skeleton any more anxious than he already was.)

 

Sans snickered.

 

“pfft. you said coitus.”

 

Grillby lightheartedly laughed. He looked at Sans expectantly, who sat there clueless for a couple of seconds before catching on.

 

“... oh! o-oh… yeah, i uh,” Sans looked his clothed self over with an unsatisfied expression and moved to remove them before stopping hesitantly and looking everywhere else in the room.

 

“i don’t… think you want…” he faltered. “i can’t imagine bare bones are too comfortable to be, uh, i don’t… know if…”

 

Sans nervously glanced away, wringing his hands together as he stuttered and racked his cranium for any excuse to not expose himself.

 

Grillby slowly brought his hands to Sans’ face and tilted it upwards, smiling at how the skeleton nuzzled into the heat.

 

“Sans, do you remember what I said?”

 

“...we, uh, we can stop whenever i want?”

 

“Well yes, that too. But before that. I said you are much more magnificent than you believe yourself to be.”

 

“i’m just not normal, grilbs. they’re so… ugly, i’m so gross-”

 

“I can prove to you otherwise. May I?”

 

“... o-okay.” The skeleton relaxed and reluctantly slid off his jacket.

 

Rather than a pristine white, they were more of a faded, pale gray. His ribs were very wide set, and some of them were crooked here and there. Strange scars were scattered throughout his ribcage and it rose and fell steadily with every nervous breath that Sans took.

 

Sans must have taken Grillby’s silence as a negative reaction, as he reached to replace his jacket.

 

“see, i told you. they’re gross.”

 

Grillby briskly swatted the jacket out of Sans’ hand before folding it and placing it beside his own clothes on the bedframe.

 

“And who ever told you such a ridiculous fib?”

 

The room fell uncomfortably silent, and Sans went still.

 

“...It was that scientist, wasn’t it?”

 

After a moment, Sans sighed and nodded weakly. He went to curl into himself, but Grillby gently stopped him and pulled him into another comforting embrace, his soul thrumming happily when Sans relished his touch.

 

“Sans, you’re absolutely breathtaking. Don’t ever let anyone tell you otherwise… may I?” The tall monster gestured to the exposed ribs.

 

“... hmm? i guess, i mean, if you really want to.”

 

“Would you like me to?”

 

“i… y-yeah.”

  
Sans was shaking again. Less out of anticipation, and more out of fear, Grillby guessed. Fear of judgment. His teary-eyed gaze wandered everywhere but the fire monster.

 

Grillby caressed the skeleton monster’s cheeks (they were on fire) and slowly drew him into another gentle kiss. Anything to relax him and make him feel comfortable right now. His jittery bones visibly slackened as he eased into the kiss the best he could, and sharply drew in his breath when Grillby began to gently explore his ribs.

 

The only thing the bartender could compare them to was warm porcelain, despite the somewhat grainy texture along the edges and rough ridges. All Sans could feel was a soft, persistent pressure - combined with the magic-mingling kiss and the powerful chills Grillby’s hands sent shooting through the rest of his body, he whined and stilled completely. If there was a switch that turned him into a completely useless, drooling ragdoll, this was definitely it.

 

Suddenly, the relaxation morphed into anticipation and desperation, and something akin to fire crept through the entirety of Sans’ being, pun not intended. A feral need swept through him and he wasn’t aware that he was panting until Grillby broke off the kiss and focused solely on his ribs. His fingers firmly explored every inch of them before grasping his spine and rubbing the columns liberally, and Sans damn near screamed.

 

“hnngh! h-hotstuff i- hhn, wow, i mean, i’m not - this is ‘hot’ and all, but-”

 

“Would you like me to stop, Sans?”

 

“i- no! i want to do this, it’s just that… n-not to seem all hot and bothered, but-”

 

“Oh my god.”

 

“i don’t want to mess this up, it feels like i’m not in control of myself-like i’m not me, is that normal? am i supposed to feel that way, or-”

 

“You’re overthinking. Do not worry. Soon enough, this will all come naturally. You just need to stop stressing yourself out about it and take things as they come.”

 

Heh. Grillby made another one. He was silently cracking himself up.

 

“s-sorry, grillby.”

 

Grillby placed a kiss on his forehead and hugged him once more.

 

“There’s nothing to apologize for. Would you like to take a break? We can always try again later, when you’re more relaxed.”

 

“but then it just feels like i’m being unfair to you. i mean, you seemed to be enjoying yourself and i already said yes, so-”

 

Grillby shook his head adamantly.

 

“This is about you, Sans, not me. And you can change your mind anytime, one yes is not a free-for-all pass, you know. I do not want to overstep any boundaries or make you uncomfortable if you’re allowing me to do this with you.”

 

“gee, so gentlemanly, grillbs.”

 

“I do try, yes.” Grillby nodded jokingly before looking down at the bright blue glow beneath Sans’ shorts.

 

“...Sans?”

 

“yo.”

 

“Do you remember what I said about monster arousal and the buildup of magic while in heat?”

 

“eh, kinda. not gonna lie here, i’m a pretty shitty listener. please don’t quiz me on any of this later.”

 

“And how do you feel as of right now?”

 

“hot and groggy, and a little exhausted to be honest. why do you ask?”

 

Grillby smirked a little wider and gestured down at Sans’ shorts, and he glanced down at himself curiously.

 

“...what the fuck.”

 

The fire sprite burst into uncontrollable laughter as sans looked down at his crotch questionably.

 

“what the actual fuck. grillby why are you laughing i’m dying what the f u c k.”

 

“Oh hush, you’re not dying! You’re just incredibly horny! Here, c’mon let me see.” Grillby giggled madly.

 

“yeah yeah alright ya horn dog gimme a sec,”

 

Albeit reluctantly, Sans clumsily managed to wriggle out of his shorts, curiously glancing down at the accumulation of magic at his pelvis. Swirling shades of sky blue and indigo collected itself into a solid faux-skin barrier between his legs and imitated (or at least, what Sans thought they imitated) human-esque reproductive organs.

 

“whoa, that’s kinda cool,” Sans marvelled, very carefully running his bony digits along the folds of his makeshift vagina while he looked it over with a glint of curiosity in his eye.

 

“...so do you think catty gets offended when other monsters call her a pussy, or does she refer to herself as that?”

 

“What? Is that really what you’re thinking about right now?”

 

“i mean, i thought ‘pussy’ and one thing led to another, y’know? why, is that racist? or speicist or something? don’t tell her i said that okay? just to be safe.”

 

Again, Grillby found himself laughing much harder than he thought possible, throwing a hand over his mouth and shaking his head aggressively.

 

“i wonder if she calls catnip ‘meowijuana’.”

 

“Oh my God, stop!”

 

“sorry, this pussy got me off track,” Sans giggled in response. “what were we doing?”

 

“You,” The flame monster snickered as he playfully pushed Sans back against the bed and resuming his exploration.

 

“careful there grillbs, curiosity killed the ca-mpph!”

 

The bartender explored the bones beneath Sans’ quivering legs, kissing him so roughly that Sans found himself completely pressed against the bed, and while his valiant efforts to kiss back with equal force were pretty much futile, they were damn cute to witness.

 

The skeletons eyes widened at the unfamiliar feeling, letting out a squeak of surprise as Grillby’s hands brushed against his newly formed entrance. The burning, needy sensation that boiled beneath his bones had increased tenfold and now radiated from where his magic had pooled between his legs. It was a foreign feeling. Unbearable heat spread throughout his bones and sweat collected at his temples. This new onslaught of sensitivity and the strange, hungry hollowness that followed sent a jolt of fear and anxiety up his visibly shivering spine. Panic suddenly gripped him and the small monster clung onto Grillby with all his (meek) might.

 

“ah! grilbs! wha- i- w-wait, what’s th- what, i-i don’t-”

 

Grillby quickly drew away with a chuckle. “Relax, Sans. Take a deep breath. That’s just your magic. It doesn’t hurt, does it?”

 

“n-no, it just- fuck, i don’t know! it just, it feels- why’d you stop?”

 

“I do apologize,” Grillby chuckled kissed him once more, a little fiercer this time, which Sans thought was impossible but was quickly proven wrong. The fire monster found that the more aggressive he was with the skeleton, the more positive reactions he received - which, of course, made complete sense, seeing that he was in heat. Or maybe Sans was just a natural born sub? He slipped his hand back down and slowly stroked the entrance, smirking at how wet Sans was.

 

Sans visibly tensed at the touch.  

 

“i-is it broken or something?” He asked in a serious tone, pulling away from Grillby’s harsh kisses.

 

__Jesus fucking christ that was so fucking cute-_ _

 

“No, not at all. It’s absolutely perfect, Sans.” Grillby kissed the blushing skeleton’s forehead. “You’re absolutely perfect in every way, you know that?”

 

The petite monster visibly relaxed, and with a newfound confidence, pulled Grillby’s face down into another kiss. Grillby happily complied.

 

“... But, perhaps I should investigate anyways. After all, it is better to be safe than sorry.”

 

Sans gave him a questioning look, and Grillby merely smiled back mischievously.

 

The translucent blue substance steadily dripped from the magic opening, making Grillby’s next move quite easy. He slowly pressed a single finger inside, and Sans froze, inhaling deeply. Sans was a rather small monster, seeing that he was a good foot and a half shorter than the flaming elemental above him. The unexplored crevice reacted immediately, tightening and readjusting itself to the unexpected invasion. Grillby was sure that if Sans’ fingertips had held onto him any tighter, he would have cut him open.

 

Said skeleton monsters eyes were clenched shut, and he was breathing deeply, greedily taking in gulps of air in a desperate effort to hold himself together and stay calm.

 

“Can you look at me, Sans?”

 

“... i g-gotta hand it to you, grillbs,” Sans breathed shakily, shooting him the most confident, snarky look he could muster. “you’re pretty handy to have around when i get like this. i-i know i’m in - hngh - real good hands.”

 

“What makes you feel the need to pun at a time like this?”

 

“what? can’t hand-le them, huh? h-heheh.”

 

Grillby smirked.

 

“I could say the same for you, you know.”

 

Without warning, he curled his finger and began roughly plunging upward. Sans jumped, first out of surprise, and then again, out of something else entirely. He had slightly opened his mouth to say something but was cut off by the sudden pressure inside of him that made Sans lose his train of thought completely. He thought about saying a pussy pun but it was quickly forgotten. His mouth stayed open like that, much to Grillby’s amusement. Sans gradually went slack, save for the magic muscle that kept unconsciously bucking into Grillby’s hand, and he whined loudly at the pleasurable sensation the elemental was currently causing him.

 

Sans’ felt his fear and unease melt away into bliss as the flame monster continued to press into him into that spot repeatedly. His eyes had become unfocused pinpricks of bright, flickering light that occasionally rolled up into the back of his head before recentering themselves onto Grillby’s red flushed face.

 

“It’s good to know that to get you to stop making puns, I could simply use a more hands-on approach. So, how does it feel, Sans?”

 

“f-fuck,” He choked out a low, ragged moan.

 

“I’m sorry? I missed that, I can’t very well hear you.” Grillby chuckled devilishly, lessening the intensity of his ministrations, which certainly got Sans’ attention.

 

“g-good! it’s so fucking good grillby,don’t- please don’t stop- please, i need it, please, come on already, fuck!” Sans pleaded loudly, desperately thrusting himself against Grillby’s partially-inserted finger in an attempt for further friction, ultimately losing himself in the satisfaction.

 

Grillby slowly inserted two more fingers and Sans groaned.

 

“Oh, just look at you… you’re so beautiful, Sans,” the fire elemental marvelled to himself as Sans continued to twitch and writhe in pleasure beneath him, clutching his sheets in his shaky digits while Grillby continued to wriggle inside him. The dripping blue shaft clenched and tightened around his fingers needily, and his other hand occupied itself with exploring every inch of the skeleton, experimentally stroking and pressing certain bones. Breathy, low moans echoed throughout the room.

 

And then the sensation disappeared, Grillby yanking his fingers out of Sans with a wet pop. The skeleton damn near sobbed.

 

“grillbyyyyy, noooooo…”

 

“Did you like that?”

 

“oh no, not at all, godawful, i just said no to make it ironic, you know?”

 

“Sans I swear to God-”

 

“but hey, i feel a lot better though, so why did you stop?”

 

“Because I have something better for you.”

 

“...oh… you do, huh?” Sans huffed snidely with a playful glint in his eye, flinging his feet off either side of the bed and spreading his legs out widely. His face was almost completely flushed and he had a stupid grin on his face.

 

“Of course. I wouldn’t just tease you and not give you the grand finale, what kind of monster would I be then?”

 

“heh. y’know,” The skeleton monster commented to himself as Grillby gradually (and carefully) removed his pants, “i might not have ever seen stuff like this except for in those animes that Alphys shows me, but you’re pretty damn hot.”

 

“Yes, perhaps Alphys should lay off on the ‘animes’ now that you have the real thing- Sans if that was a pun I swear.”

 

“nah, they’re not too bad, honestly. good for studying human anatomy, though since i don’t have many books on ‘em.”

 

“Such a nerd,” Grillby laughed, clambering onto the bed above him and standing up on his knees, stroking his shaft eagerly. The sight made the magic in Sans’ bones rattle.

 

“what are you waiting for? fuckin wreck me.” Sans spat.

 

“Believe me, I fully intend to.”

 

-

 

Four hours later, pleasure racked Sans’ small frame for the countless time that night as he came around the elemental’s length, which was enough to send Grillby over the edge himself. Blue tears streaked down his face while the overstimulation and overbearing satisfaction shook through him. Both their breath’s hitched and they rode out their high together before finally collapsing into one another.

 

Grillby sat up eagerly.

 

“So, how are you feeling, Sans? Do you think you need one more round? … Sans?”

 

The skeleton lazily grunted in response, not even bothering to open his eyes.

 

“... ...hmm?”

 

“I asked you how you are feeling.”

 

“... hmm?”

 

Grillby chuckled as he saw the faux genitalia redisperse its magic throughout his body, and watched the indigo flush gradually fade from Sans’ face.

 

“I believe that your heat has been properly dispelled for now.”

 

“no… one more… just one, c’mon-”

 

“Aren’t you sore?”

 

“... no.”

 

“You’re a terrible liar.”

 

“...m’so tired, grillbs, jesus christ.”

 

Grillby snorted and caressed the skeleton’s tearstained cheeks.

 

“Yes, you are quite small. I can imagine that would take a lot out of you.”

 

Sans’ chuckled lowly. “heh. coitus.” He laughed with a broken voice, sore from screaming and moaning earlier.

 

“Why is that word so funny to you? I don’t understand.” Grillby said. He pulled the shirt back over his head and reached forward to grab Sans’ jacket before a bony hand caught him at the wrist and gave an eager tug.

 

“please d-don’t go, okay?” Sans pleaded.

 

And with that, Grillby crawled beside him and flung the comforter over the both of them.

 

“I’m not going anywhere, Sans.”

**Notes for the Chapter:**

> Jokes on you Im already burning in hell lmao bye felicia
> 
> So basically I have this whole headcanon/theory about The Underground Sexy Times (™).  
> Originally, not only is Sans to lazy to bate/bang, but he has also grown up believing it to be impossible for skeletons to do so, probably based on Gaster’s bs theory about not having any inherent sexual organs or skeleton dna or something. So whenever Papyrus got like that Sans would take care of it as if it were a fever and tell him that sometimes skeleton magic gets too low to the point where they start to overheat (when in actuality, it’s the opposite; there’s a massive build up of horny ass magic that grows regardless of whether or not you use your magic amply, and can only be properly channeled/gotten rid of via the Fucc). I think that after the war, that all monsters magic (as a whole/it’s own entity) synced and decided to up the whole fucking situation in order to make up for the loss of population. Ie, more horny monsters = more monster kids.  
> I like to think, because I’m a gross pos, that if a group of monster’s heat cycles/buildup of magic are synced, that a massive fucking monster orgy ensues. Because of this I also think most of them would be naturally polyamorous.  
> I also have a soulmate theory that basically just boils down to two monsters whose traits, magic, genetics, personalities etc. all make for good offspring and equal out each others magic/hp. Sometimes it takes a while to find one or it immediately is linked right off the bat. After soul sharing multiple times it gets to the point where neither need to worry about cheating/mating with other monsters because the two souls will only willingly interact with each other. If a pair of monsters has been soul sharing for a long time but their souls still haven’t fully linked/become exclusive to each others yet, then they might not actually be soul mates. Idk the whole soul mate thing is a mysterious topic in the underground I think but its great fun to delve into with a scientific mind lmao  
> Eh im still workin out the kinks (lmao) but thats what I got so far take it or leave it friendos (if you do use it tho you should definitely credit me tho thnx).  
> Also yeah I straight up broke my toe and im sick as a fucking dog im ready for death

**Author's Note:**

> LMAO im sorry for the cliffhanger but i dont think imma continue this one until i see if people are reading it, so if you want it finished, leave a kudos and a comment, any suggestions/requests are welcome! But I probly won't be able to take all of them, so the ones i like the best i guess. Idk when imma be able to finish this up, ive been real busy, but stay determined!  
> Later nerds.


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